Mitochondrial Ca 2+ Uptake Relieves Palmitate-Induced Cytosolic Ca 2+ Overload in MIN6 Cells

• Published in: Molecules and cells Vol. 43; no. 1; pp. 66 - 75
• Main Authors: Ly, Luong Dai, Ly, Dat Da, Nguyen, Nhung Thi, Kim, Ji-Hee, Yoo, Heesuk, Chung, Jongkyeong, Lee, Myung-Shik, Cha, Seung-Kuy, Park, Kyu-Sang
• Format: Journal Article
• Language: Korean
• Published: 2020
• Subjects: mitochondrial $Ca^{2+}$ uniporter; lipotoxicity; pancreatic ${\beta}$-cell; cytosolic $Ca^{2+}$ overload; oxidative stress
• ISSN: 1016-8478; 0219-1032

Saturated fatty acids contribute to β-cell dysfunction in the onset of type 2 diabetes mellitus. Cellular responses to lipotoxicity include oxidative stress, endoplasmic reticulum (ER) stress, and blockage of autophagy. Palmitate induces ER Ca2+ depletion followed by notable store-operated Ca2+ entry. Subsequent elevation of cytosolic Ca2+ can activate undesirable signaling pathways culminating in cell death. Mitochondrial Ca2+ uniporter (MCU) is the major route for Ca2+ uptake into the matrix and couples metabolism with insulin secretion. However, it has been unclear whether mitochondrial Ca2+ uptake plays a protective role or contributes to lipotoxicity. Here, we observed palmitate upregulated MCU protein expression in a mouse clonal β-cell, MIN6, under normal glucose, but not high glucose medium. Palmitate elevated baseline cytosolic Ca2+ concentration ([Ca2+]i) and reduced depolarization-triggered Ca2+ influx likely due to the inactivation of voltage-gated Ca2+ channels (VGCCs). Targeted reduction of MCU expression using RNA interference abolished mitochondrial superoxide production but exacerbated palmitate-induced [Ca2+]i overload. Consequently, MCU knockdown aggravated blockage of autophagic degradation. In contrast, co-treatment with verapamil, a VGCC inhibitor, prevented palmitate-induced basal [Ca2+]i elevation and defective [Ca2+]i transients. Extracellular Ca2+ chelation as well as VGCC inhibitors effectively rescued autophagy defects and cytotoxicity. These observations suggest enhanced mitochondrial Ca2+ uptake via MCU upregulation is a mechanism by which pancreatic β-cells are able to alleviate cytosolic Ca2+ overload and its detrimental consequences.